If you have an external programmer (e.g. an AVR-ISP, STK500, or parallel programmer), you can burn sketches to the Arduino board without using the bootloader. This allows you to use the full program space (flash) of the chip on the Arduino board. So with an ATmega168, you'll get 16 KB instead of 14 (on an ATmega8 you'll get 8 KB instead of 7). It also avoids the bootloader delay when you power or reset your board.
This can be easily done in this way:
In order to come back to the default way to program your Arduino you have to rewrite the bootloader. To do this: